This invention relates to nuclear reactor constructions of the kind comprising a nuclear fuel assembly submerged in a pool of coolant contained in a primary vessel and is primarily directed to liquid metal cooled fast breeder nuclear reactor constructions of that kind.
In a liquid metal cooled fast breeder nuclear reactor construction the fuel assembly is submerged in a pool of liquid metal coolant, for example, sodium, within a primary vessel which is housed within a concrete vault. The primary vessel is suspended from the roof of the vault and the roof has apertures for access to the primary vessel. One large access aperture is closed by a rotatable plug which carries apparatus for servicing the fuel assembly and other apertures are occupied by heat exchangers and pumps for effecting transfer of heat from the fuel assembly to external steam generating apparatus. The rotatable plug carries a core cover structure which comprises a generally cylindrical shell having lower and intermediate grid plates for tubes which serve to shroud the control rods and instrumentation connections. The core cover structure also provides a hold down facility for the fuel assembly in an emergency and the intermediate grid plate diverts coolant flowing upwardly from the fuel assembly into radial flow towards the heat exchanger inlet ports. During operation of the nuclear reactor the coolant leaving the fuel assembly is subject to rapid changes in temperature and to avoid thermal shock to the lower and intermediate grid plates (which are submerged in the pool of coolant) and the shell the grid plates and shell are perforated so that the coolant can flow freely in and about the structure and on both sides of the grid plates. However, the upper surface of the intermediate grid plate is submerged in a region of the pool of coolant which is relatively static and at a more constant temperature and therefore is more subject to thermal shock due to temperature differential across it. Thus one object of the invention is to reduce the temperature differential across the intermediate grid plate of the core cover structure and thereby reduce its subjection to thermal stresses.